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Internet Engineering Task Force (IETF)                       A. MelnikovRequest for Comments: 5803                                 Isode LimitedCategory: Informational                                        July 2010ISSN: 2070-1721Lightweight Directory Access Protocol (LDAP) Schema for Storing SaltedChallenge Response Authentication Mechanism (SCRAM) SecretsAbstract   This memo describes how the "authPassword" Lightweight Directory   Access Protocol (LDAP) attribute can be used for storing secrets used   by the Salted Challenge Response Authentication Message (SCRAM)   mechanism in the Simple Authentication and Security Layer (SASL)   framework.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5803.Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Melnikov                      Informational                     [Page 1]

RFC 5803          LDAP Schema for Storing SCRAM Secrets        July 2010Table of Contents1. Overview ........................................................22. Conventions Used in This Document ...............................33. Security Considerations .........................................34. Acknowledgements ................................................45. Normative References ............................................41.  Overview   This document describes how the authPassword LDAP attribute   [AUTHPASS] can be used for storing secrets used by [SCRAM] Simple   Authentication and Security Layer [RFC4422] Mechanisms.      The "scheme" part of the authPassword attribute is the SCRAM      mechanism name (always without the "-PLUS" suffix), e.g., "SCRAM-      SHA-1".  See [SCRAM] for the exact syntax of SCRAM mechanism      names.      The "authInfo" part of the authPassword attribute is the iteration      count (iter-count in the ABNF below), followed by ":" and base64-      encoded [BASE64] salt.      The "authValue" part of the authPassword attribute is the base64-      encoded [BASE64] StoredKey [SCRAM], followed by ":" and base64-      encoded [BASE64] ServerKey [SCRAM].   Syntax of the attribute can be expressed using ABNF [RFC5234].  Non-   terminal references in the following ABNF are defined in either   [AUTHPASS], [RFC4422], or [RFC5234].       scram-mech     = "SCRAM-SHA-1" / scram-mech-ext                      ; Complies with ABNF for <scheme>                      ; defined in [AUTHPASS].       scram-authInfo = iter-count ":" salt                      ; Complies with ABNF for <authInfo>                      ; defined in [AUTHPASS].       scram-authValue = stored-key ":" server-key                      ; Complies with ABNF for <authValue>                      ; defined in [AUTHPASS].       iter-count   = %x31-39 *DIGIT                      ; SCRAM iteration count.                      ; A positive number without leading zeros.       salt         = <base64-encoded value>Melnikov                      Informational                     [Page 2]

RFC 5803          LDAP Schema for Storing SCRAM Secrets        July 2010       stored-key   = <base64-encoded value>                      ; See definition in [SCRAM].       server-key   = <base64-encoded value>                      ; See definition in [SCRAM].       scram-mech-ext = "SCRAM-" 1*9mech-char                      ; Other SCRAM mechanisms registered                      ; in the IANA registry for SASL                      ; mechanism names.       mech-char    = <Defined inRFC 4422>   Note that the authPassword attribute is multivalued.  For example, it   may contain multiple SCRAM hashes for different hashing algorithms.2.  Conventions Used in This Document   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this   document are to be interpreted as described in [RFC2119].3.  Security Considerations   This document defines how the authPassword attribute can be used to   store SCRAM secrets.  Therefore, security considerations relevant to   [SCRAM] and hash functions used with it are also relevant to this   document.   General security considerations related to the authPassword attribute   (as specified in [AUTHPASS]) also apply to the use of authPassword as   specified in this document.  In particular, the values of   authPassword SHOULD be protected as if they were cleartext passwords.   A read operation on this attribute that is not protected by a privacy   layer (such as IPsec or TLS) can expose this attribute to an attacker   who a) would be able to use the intercepted value to impersonate the   user to all servers providing SCRAM access using the same hash   function, password, iteration count, and salt or b) would be able to   perform an offline dictionary or brute-force attack in order to   recover the user's password.   Servers MUST validate the format of the authPassword attribute before   using it for performing a SCRAM authentication exchange.  It is   possible that an attacker compromised the LDAP server or got access   to the entry containing the attribute in order to exploit a   vulnerability in the subsystem performing the SCRAM authenticationMelnikov                      Informational                     [Page 3]

RFC 5803          LDAP Schema for Storing SCRAM Secrets        July 2010   exchange.  Big iteration counts and invalid base64 encoding are two   possible (but not the only) exploits in the format specified in the   document.4.  Acknowledgements   The author gratefully acknowledges the feedback provided by Chris   Newman, Kurt Zeilenga, Chris Lonvick, Peter Saint-Andre, Barry Leiba,   and Chris Ridd.5.  Normative References   [AUTHPASS]  Zeilenga, K., "LDAP Authentication Password Schema",RFC 3112, May 2001.   [BASE64]    Josefsson, S., "The Base16, Base32, and Base64 Data               Encodings",RFC 4648, October 2006.   [RFC2119]   Bradner, S., "Key words for use in RFCs to Indicate               Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC4422]   Melnikov, A. and K. Zeilenga, "Simple Authentication and               Security Layer (SASL)",RFC 4422, June 2006.   [RFC5234]   Crocker, D. and P. Overell, "Augmented BNF for Syntax               Specifications: ABNF", STD 68,RFC 5234, January 2008.   [SCRAM]     Menon-Sen, A., Newman, C., Melnikov, A., and N. Williams,               "Salted Challenge Response Authentication Message (SCRAM)               SASL Mechanisms",RFC 5802, July 2010.Author's Address   Alexey Melnikov   Isode Limited   5 Castle Business Village   36 Station Road   Hampton, Middlesex  TW12 2BX   UK   EMail: alexey.melnikov@isode.com   URI:http://www.melnikov.ca/Melnikov                      Informational                     [Page 4]